The present invention relates to a process for the batch fine purification of uranium, or plutonium, or plutonium and uranium, recovered in a reprocessing process for irradiated nuclear fuel and/or fertile materials, after a coarse separation of transuranic elements in the case of uranium, or of neptunium and americium, in the case of plutonium or plutonium and uranium. In the coarse purification, fission and corrosion products and other impurities (coase purification) are also separated from the uranium, or plutonium, or plutonium and uranium. For each bath, an aqueous, nitric acid solution, which in the case of uranium is a uranyl-nitrate solution (UNH original solution), in the case of plutonium is a plutonyl-nitrate solution (PuNH original solution), and in the case of plutonium and uranium is a plutonyl-uranyl-nitrate solution (Pu/UNH original solution), and which solution still contains residual amounts of transuranic elements in the case of uranium or of neptunium and americium compounds in the case of plutonium or plutonium and uranium, and which solution still contains fission and corrosion products and their compounds and other impurities, is cooled to bring about the crystallization of purified UO.sub.2 (NO.sub.3).sub.2.6H.sub.2 O (UNH), or plutonyl nitrate, or plutonyl uranyl nitrate, from the solution and the resulting crystals are separated out of their mother liquor and washed.
The Purex process has been carried out world wide for the reprocessing of spent nuclear fuels. This process and its variants are thoroughly explained in the literature. In the Purex process a weight percent of up to approximately 5% radioactive fission products, transuranic elements and possibly activated impurities of U and Pu are thoroughly separated from U and Pu. Thereafter, the U and Pu are also separated from each other. The U and Pu products must be so pure that the refabrication of new fuel from the U and Pu products can be carried out without the all too costly and expensive radiation protection measures. For that reason, the removal of the .gamma.-irradiated nuclides is especially important. Further purification specifications concern inactive impurities as well, as for example iron, chromium and nickel, which are produced through corrosion of the process apparatuses which are generally built out of stainless steel, and can be introduced into the process solutions, as well as further impurities from process chemicals or other materials. Details of the purification specifications depend on selected refabrication processes, among others.
The refabrication of Pu containing fuel is expensive because, due to the high radiotoxicity of Pu, it must take place in glove boxes. The refabrication of Pu free U fuel can be achieved more simply, and without use of the box technique, if the purity of the U products is comparable with that of natural uranium. Not only is a high product purity required from a reprocessing process, but at the same time also a high product yield. Such extreme demands cannot normally be satisfied by a single chemical separation process. For the technical solution of this object, usually two separation processes are combined. In the first separation operation, a coarse purified product is obtained with high yield, as well as a waste stream which contains the impurities. This first separation is referred to as a coarse purification. In the second separation, a fine purified product is obtained in low yield, as well as a waste stream, which contains the rest of the impurities and some product. This second separation is referred to as fine purification.
An extraction cycle in the Purex process already contains such a combination of coarse and fine purification operations.
Coarse Purification:
In the coarse purification, U and Pu are completely extracted from a nitric acid fuel solution in a first extractor with an organic auxiliary (extractant) agent phase that is immiscible either with water or with aqueous solutions, e.g. 30% tributyl phosphate (TBP) as an extractant dissolved in dodecane, to form an organic product solution and a nitric acid waste stream. The nitric acid waste stream contains highly active fission products (SP), as well as transuranic elements (TU), and possibly corrosion products (KP), or other impurities (VP), but practically no uranium and/or plutonium. The organic product solution, which carries uranium and plutonium with it, still contains residues of SP, TU, KP and VP, and must therefore be further fine purified.
Fine Purification:
In the fine purification, the residual impurities are washed out of the organic product solution with pure, aqueous nitric acid in a second extractor. Because again some U and Pu product are washed out by that process, the used wash acid must be fed back into the fuel solution before the first extractor.
The purified U and Pu products resulting from the washing must once again be extracted into a nitric acid aqueous phase from the organic product solution for reprocessing. First only the Pu is recovered and then the U is recovered. In this way, the two products can be separated from each other. The extraction of P and U into nitric acid aqueous phases completes the first extraction cycle of the Purex process. Technical performance of the Purex process, however, is not yet sufficient after one extraction cycle. In order to obtain a product purity according to specification, the uranium product as well as the Pu product must be fine purified by one to two additional extraction cycles.
By improving the fine purification steps, attempts have been made to obtain a sufficient product purity after only the first extraction cycle. In this manner, the washes of the organic product solution were conducted not only in one, but rather in several fine purification steps, and under optimum operating conditions (wash acid concentration, operating temperature, etc.). That such efforts were, up to now, not too successful as a whole is most clearly seen by the fact that most new installation designs are based on two to three extraction cycles. Conducting a continually operating extraction cycle with pulse columns lasts about half of a day, depending on the size, wherein at the beginning defective batches can be produced. Frequency operating interruptions are therefore not desired.
The operating conditions of individual extraction cycles differ. The reason for that is not least the undesirable behavior of individual problem elements in the Purex system. Particularly the fission products Ru(Rh), Zr(Nb), Tc as well as Np can reach the desired product in disturbing proportions, because, under the usual process conditions, they are extracted with the desired product in more or less large proportions. Because there are no operating conditions by which a co-extraction of disturbing amounts of all problem elements can simultaneously be suppressed, each problem element must be considered one after another in the individual extraction cycles by operating conditions correspondingly determined. The entire process is thereby made more complex.
Also, with altered operating conditions in the individual extraction cycles, repeating the similar separation operation 2 or 3 times is not optimum for an effective chemical separation.
Auxiliary materials are often used to conduct chemical separations more simply. An aqueous phase is most often used as an auxiliary agent, in which, for example, a series of different materials is dissolved. For material separation, one can, for example, precipitate a solid precipitate by addition of certain materials, in which one or several components from the solution are strongly enriched. The actual separation operation is the separation of the solid or fluid phase, for example by filtration. In the Purex process, in addition to the aqueous agent phase, yet a second, organic agent phase is needed, which is immiscible with the aqueous phase, because of the separation by a liquid extraction. For the material separation, then, operating conditions are regulated under which the products are strongly enriched either in the one or in the other phase. Not only is the subsequent separation of the two immiscible liquid phases simple, but also the technical handling of liquids in further operations, such as conveying, dosing, storing, etc. That is a particular advantage of the Purex process, because due to the radioactivity of the materials, it must be carried out by remote control in hot cells.
Nevertheless, the use of an organic solvent has certain disadvantages. The combustibility must be controlled by additional combustion protection measures. The formation of explosive organic compounds ("red oil") in the process evaporator for nitric acid solutions must be avoided by additional measures. The organic solvent is slowly disintegrated by the radiation. The disintegration products must be removed by additional purification equipment, e.g. for washing with soda solution, whereby radioactive waste is produced and product losses can appear in small amounts. Very large solvent volumes would be needed in the installation, because the product concentration in the solvent is usually small (e.g. 0.3 mol/l U). A corresponding space in expensive hot cells is needed for the handling and storage of such a large amount of solvent. If pulse columns are used as extraction apparatuses, the hot cells must, moreover, have a great height of construction.
In a well conducted first Purex extraction cycle, already more than 99.9% of the SP is removed, aside from a few problem elements. If one extrapolates from about 5% that is originally present, then one finds a total of less than 100 ppm in the aqueous U and Pu product solutions of the first extraction cycle according to size. For the further separation of these radioactive trace impurities from a chemical outlook, it is not advantageous to use large amounts of organic solvents for the main products U or Pu, respectively.